Field of the Invention
The present invention relates to a focus adjustment apparatus equipped with a phase-difference detection method and a contrast detection method.
Description of the Related Art
In imaging apparatuses typified by a single-lens reflex camera, a method for capturing an image while viewing a live view (LV) screen has recently been gaining in great importance. Demand for moving image photographing has been growing in particular. In moving image photographing, it is important that photographing can be performed while viewing a LV screen in a comfortable way.
Various techniques have been discussed as an automatic focusing (AF) method of an imaging apparatus while performing LV. One of the main techniques is a contrast detection method. In the contrast detection method, a contrast evaluation value is generated from an imaging signal obtained by using an image sensor while moving a focus lens. A focus lens position that maximizes the contrast evaluation value is searched for to obtain an in-focus state.
However, according to the contrast detection method, it is not possible to determine with ease the focusing position and direction to put an object into a focus. As a result, the contrast detection method may take on unfavorable behavior, such as taking a long time to focus, mistaking the focusing direction, and passing over the in-focus position. In the moving image photographing in particular, unfavorable behavior is undesirable because all the focusing operations are recorded.
AF methods capable of favorable focusing even during LV photographing have been discussed. Among such methods is an imaging surface phase-difference detection (imaging surface phase-difference AF) method which is a phase-difference detection method performed on an image sensor surface. According to the phase-difference detection method, a defocus amount is calculated from a phase difference between a plurality of image signals obtained by receiving light fluxes which have passed through different exit pupil areas of a photographing optical system. The focus lens is moved by an amount of movement corresponding to the defocus amount, whereby an in-focus state is obtained.
As an imaging surface phase-difference detection method, Japanese Patent Application Laid-Open No. 2001-083407 discusses a method for arranging a plurality of photoelectric conversion elements under the same microlens, in an imaging pixel of an image sensor. In such a configuration, the plurality of photoelectric conversion elements receives light passed through respective different exit pupil areas, whereby focus detection can be performed simultaneously with imaging. Specifically, the outputs of the plurality of photoelectric conversion elements can be compared to perform phase-difference detection AF. By employing the imaging surface phase-difference detection method, even in LV photographing, AF using the phase-difference detection method can be carried out which enables high speed and favorable focusing.
As an imaging surface phase-difference detection method, Japanese Patent Application Laid-Open No. 2013-025129 discusses a configuration in which phase-difference detection AF is performed by arranging a focus detection pixel in an image sensor. Japanese Patent Application Laid-Open No. 2013-025129 further discusses a system combining the imaging surface phase-detection method with a contrast detection method to compensate disadvantages of contrast AF and phase-difference AF and thereby achieve AF of higher quality. For example, according to Japanese Patent Application Laid-Open No. 2013-025129, wobbling driving is performed if no focus detection result by the phase-difference AF is obtained. The focus lens is driven based on a calculation result of either the phase-difference AF or the contrast AF, whichever is earlier obtained.
However, a method for driving a focus lens based simply on a first-calculated result, like Japanese Patent Application Laid-Open No. 2013-025129, has a possibility to erroneously determine the driving direction of the focus lens. For example, if the focus lens is far from the in-focus position (a large blur), the image signals of the imaging surface phase-difference AF tend to become asymmetric. This increases the possibility of failing to identify a clear peak of image matching and calculating an erroneous result. In such a case, even if the result of the phase-difference AF is calculated first, the use of the result of the phase-difference AF carries a high risk and the contrast AF is likely to detect a more reliable focusing direction. However, the method discussed in Japanese Patent Application Laid-Open No. 2013-025129 is not able to determine an appropriate detection result between the phase-difference AF and the contrast AF according to the photographing situation.
As one of characteristic controls when capturing a moving image, the focus lens is stopped after coming into focus and drive again (restart) the focus lens when an object moves or a scene changes. The control of not driving the focus lens when a focus adjustment is not needed can prevent unnecessary focusing. The same applies when AF control is performed by using the phase-difference detection method.
FIGS. 27A and 27B illustrate a photographing scene where a person, a tree, and a mountain are included as objects. The objects illustrated in solid lines represent an in-focus state. The objects illustrated in broken lines represent an out-of-focus state. The left half of FIG. 27A illustrates a state where all the objects on the screen are out of focus. In such a case, the focus should be immediately adjusted to objects (person and tree) as illustrated in the right half. The left half of FIG. 27B illustrates a state where some objects (person and tree) are already in an in-focus state. In such a case, as illustrated in the right half, the focus lens should be controlled to stop without a restart if the scene does not change.
Japanese Patent Application Laid-Open No. 2012-128316 discuses a method for performing a restart determination while the focus lens is stopped, based on focus detection information of a phase-difference detection method obtained by an exterior measurement sensor, focus detection information of a contrast detection method, and waveform information of the phase-difference detection method.
According to Japanese Patent Application Laid-Open No. 2012-128316, the focus lens is immediately restarted once the focus detection result of the phase-difference detection method exceeds a determination threshold when an AF evaluation value varies. For example, the AF evaluation value can vary when another object comes into the detection area of the AF evaluation value. As a result, even if the main object is in the in-focus state at the current focus position, the focus lens can be needlessly restarted to cause blurring. Further, the AF evaluation value is less likely to change in low-luminance situations. In such situations, by the determination method according to Japanese Patent Application Laid-Open No. 2012-128316, the focus lens may fail to restart even if the scene changes.